Every time the heart beats, electrical activity is generated. This activity, called the electrocardiogram (EKG) is the result of cellular depolarization which in turn causes the cardiac muscle cells to contract. The EKG signal from a single cardiac cycle is shown in FIG. 1. It consists of a P wave, which results from depolarization of the atria (upper receiving chambers of the heart), a QRS complex, which results from the normal, coordinated, depolarization of the ventricles (lower, thick, muscular pumping chambers), and a T wave, which is caused by ventricular repolarization. Depolarization originates in specific atrial tissue called the "sinus node", passes through the atria, then through specific junctional tissue between the atria and ventricles called the "atrioventricular node" (FIG. 2). The signal then passes through specialized conduction tissue which delivers the depolarization activity to both the left and right ventricles simultaneously. This results in a narrow QRS complex, usually less than 10 msec in duration. If the pacemaker "fires" regularly, this results in "normal sinus rhythm". Such a normal rhythm is shown in FIG. 3.
Any variation from normal sinus rhythm may be broadly referred to as an "arrhythmia". For example, if atrial tissue other than the sinus node spontaneously depolarizes and initiates a cardiac contraction, a "premature atrial contraction" (PAC) will result producing a characteristic change in the EKG (FIG. 4). Note that the P waves in the premature beats occur early and are shaped differently than the sinus P waves, as they have originated from a different "ectopic" location. As a general rule, these PAC's do not have serious clinical consequences.
PAC's have normal QRS morphology, as the derangement (abnormality or ectopic beat) has occurred entirely above the ventricles. By contrast, ventricular ectopic beats radically alter the shape of the QRS complex (FIG. 5). This is due to the fact that when depolarization originates from an ectopic ventricular focus, the electrical activity does not pass through the specialized conduction tissue, and arrives at the two ventricles spread out over time. Thus, such complexes are generally broad and slurred as shown.
A single premature ventricular contraction (PVC) is generally not of clinical significance. However, if many such beats occur together, they may compromise cardiac function. Furthermore, if many PVC's are occuring from many different sites in the ventricle (multifocal PVC's) this may indicate a serious global derangement of ventricular activity which may constitute a medical emergency. A number of important arrhythmias and their corresponding EKG patterns will be briefly described.
FIG. 6a shows multiple unifocal PVC's. Note that the shapes of the ectopic beats are uniform. When such beats occur in a regular pattern they may be referred to with specific terminology. For example, "bigeminy" is when every other beat is abnormal (FIG. 6b). "Trigeminy" is when every third beat is abnormal (FIG. 6c). When two such beats occur in a row it is called a "couplet" (FIG. 6d). When they occur three or more in a row they are referred to as "a run of ventricular tachycardia" (FIG. 6e).
When PVC's originate from more than one location, they are called "multifocal" and the corresponding EKG has abnormal complexes which differ from one another. Such "multiform" PVC's are shown in FIG. 6f. If every other beat is a multiform PVC this results in "multiform bigeminy" (FIG. 6g). If multiform PVC's occur one after another, this constitutes a "run of multiform ventricular tachycardia" (FIG. 6h).
Each of these rhythm disturbances has its own clinical significance. Therefore, it is important for the physician to identify and categorize the arrhythmia. Currently, this is done by providing the physician with a graphic representation of the EKG tracing which he or she may analyze. Such tracings can be either provided as hard copy or made available as a stored tracing on a monitor screen. It is also common to record the EKG signal for 24 hours and provide the physician with excerpts from such a stored waveform for detailed analysis. This is referred to as a "holter monitor recording."
There are obviously situations in which it is desirable to execute such rhythm analysis immediately. Furthermore, there are environments, most notably during cardiac catheterization procedures, when it is not convenient for the physician to constantly look at a monitor. Thus, it would be desirable to provide useful information about these arrhythmias to the physician by a distinct audible signal, whereby the nature of the signal not only indicates the presence of an arrhythmia, but also provides information about the type of arrhythmia.
Automated devices have been developed to identify PVC's as well as to analyze and categorize the shape of such beats. Some of these systems apply to analysis of 24 hour "holter cardiogram" recordings, and thus do not function in real time (e.g., U.S. Pat. No. 4,316,249 to Gallant et al). Some categorize PVC's in real time and store them digitally for later analysis. One device identifies and categorizes different types of ventricular arrhythmias in real time, but does not provide an audio output to alert the physician of specific, abnormal ventricular beats (U.S. Pat. No. 4,589,420 to Adams et al). Other devices identify abnormal ventricular beats with an audible alarm, but do not provide varying outputs that reflect different types of PVC's (e.g., U.S. Pat. Nos. 4,115,864 to Vick et al; 3,881,467 to Stanley et al; 3,861,387 to Lawhorn et al). Still other devices perform an analysis of the EKG in real time, but do not provide audible alarms to alert the physician as to the results of such analysis (e.g., U.S. Pat. Nos. 4,193,393 to Schlager and 4,023,564 to Valiquette et al). Also, one device (Cardio-Beeper) produces a continuous tone whose frequency is modulated by the amplitude of the EKG waveform for transmission of an EKG over telephone. The sound emanating from this device is characteristic of the morphology of the EKG on a continuous basis, but because it produces a continuous whining sound with varying pitch, it is difficult to listen to and is not suitable for monitoring applications.
The device disclosed in this application remedies such deficiencies by providing audible output with each beat which both identifies and characterizes rhythm disturbance, thus helping the physician to become aware of both the occurrence and the nature of the arrhythmia.